The use of polymers as a rheology modifier (or viscosity modifier) or as a dispersant in an oil of lubricating viscosity is well known. Typically polymers include polymethacrylates with physical properties that have high and low temperature viscometrics as well as shear stability. The polymethacrylate polymers are generally linear and as a consequence have a set relationship of shear stability to molecular weight. Molecular weight relates to the polymer's thickening efficiency.
Attempts to prepare a star polymer from acrylic or methacrylic monomers with molecular weights and other physical properties commercially useful for a lubricating additive have been disclosed in International publication WO96/23012 A1 and European patent applications EP 979 834 A2 and EP 936 225 A1. The star polymers of WO96/23012 and EP 936 225 are prepared by anionic polymerisation techniques. It is well known that anionic polymerisation processes require carefully controlled conditions to make a star polymer. For instance the process requires highly pure solvents, an inert atmosphere substantially free of water and a low reaction temperature. The process also requires the use of alkali metal carbanionic initiators.
PCT application WO 00/02939 A1 discloses a process for preparing microgels and star polymers. The process includes atom transfer radical polymerisation and reversible addition fragmentation transfer polymerisation (RAFT). The microgel is formed from a prepolymer reacting with a multi-olefinic monomer and the product formed has a weight average molecular weight of 104 to 108.
The star polymers of EP 979 834 require from 5 to 10 weight percent of a C16 to C30 alkyl (meth)acrylate and from 5 to 15 weight percent of butyl methacrylate. A viscosity index improver with a C16 to C30 alkyl (meth)acrylate monomer present at 5 weight percent or more has reduced low temperature viscosity performance because the polymer has a waxy texture.
Processes including atom transfer radical polymerisation (ATRP) have been employed to prepare linear poly(meth)acrylate polymers, and these are disclosed in U.S. Pat. Nos. 6,391,996 B1, 6,403,745 B1, 6,403,746 B1 and 6,610,801 B1.
It would be advantageous to have a star polymer derived from a (meth)acrylic monomer with viscosity index improving characteristics and acceptable shear stability. The present invention provides a star polymer with viscosity index improving characteristics and acceptable shear stability.